1. Field of the Invention
The present invention relates to a method, a control unit, and a computer program product for detecting a gas concentration.
2. Description of the Related Art
Chemosensitive field-effect sensors are a novel technology for exhaust gas sensors, in particular for mobile exhaust gas sources. Because of the adsorptive measuring principle, the dependence of the measuring signal on the measured variable, which usually represents a concentration of a gas constituent, is sublinear rather than linear. This means, for example, that doubling the gas concentration of a certain analyte is not equivalent to doubling the measuring signal but instead only causes an increase by a smaller ratio. Based on the fact that the signal must have a certain intensity against a quasiconstant noise background, the result is that measuring certain changes in concentration, such as a change by 5 ppm in a certain gas species, is possible only up to a certain maximum concentration. At higher concentrations, only great changes in concentration by a few orders of magnitude are detectable. At even higher concentrations, the measuring signal may develop into saturation, i.e., a situation in which even a definite increase in the concentration to be measured no longer causes an increase in signal. The measurable concentration range may be shifted toward higher concentrations as a result of a change in the ambient conditions of the sensor, in particular by increasing the temperature of the sensor. The countereffect of the measure of raising the temperature is that the sensor loses precision in the range of low concentrations. In the case of mobile exhaust gas sources in particular, which may usually be operated under a wide range of different operating states, the widest possible measuring range over up to three orders of magnitude is desirable, for example, between 50 ppm and 5000 ppm, with a high measuring accuracy in the range of low concentrations at the same time.
US Patent Publication No. 2007/0278098 A1 describes a field-effect gas sensor which is based on a field-effect transistor and ensures a long operating time by minimizing the heat output. Two electrical lines are connected here to one gas-sensitive electrode, the gas-sensitive electrode being designed as a layer on the field-effect transistor, which acts as an insulating gate.